OptionParser alias behavior

Hey,

I am building a CLI tool and use the OptionParser module to parse the arguments.

Now I got a --config-override option which accepts a string. I also wanted to have a -co short version (alias). So this was my initial configuration:

    {switches, _, _} = OptionParser.parse(
      args,
      switches: [
        config_override: :string
      ],
      aliases: [
        co: :config_override
      ]
    )

But then, at runtime, I got a warning:

warning: multi-letter aliases are deprecated, got: :co

Fine, I then changed the alias to just -o:

    {switches, _, _} = OptionParser.parse(
      args,
      switches: [
        config_override: :string
      ],
      aliases: [
        o: :config_override
      ]
    )

The weird behavior I experience is, that not only -o works but also -co. Where does the -co come from?